The present invention relates to an optical recording medium and, more particularly, to a data rewritable or reversible bubble mode optical recording medium.
In data rewritable optical recording media, such as reversible optical disc memories, optical data is recorded in a recording layer, and the written data is reproduced, as required, using a data-reading light beam, or else the data previously recorded in the recording layer is erased to be replaced with new data, if necessary. Optical discs of this type are employed in numerous applications encompassing a wide variety of technical fields (including a data storage memory in a computer controlled data filing/retrieval system, a knowledge database in an artificial intelligence type "expert" system, a supplementary memory disc of a personal work station, etc.), and are considered to be one of the most promising recording media along with developments of information industries.
Magneto-optical recording media and recording media utilizing a phase transition effect are known as typical reversible optical discs (i.e., data-rewritable optical discs) to those skilled in the art. A magneto-optical recording medium has a recording layer which is made of a rare-earth-transition metal amorphous ferrimagnetic alloy film (to be hereinafter referred to as an "RE-TM film"). When a recording magnetic field is applied to the recording layer and a laser beam is focused thereonto, the perpendicular magnetization in the RE-TM film is inverted in accordance with the light intensity, and bit data is magnetically stored therein. In order to read out stored data information, the RE-TM recording layer is irradiated with a d.c.-polarized reproduction laser beam. The data is then read out by detecting the Kerr rotation of a linearly-polarized laser beam reflected by the recording layer. Since the Kerr rotation is very small, the sensitivity of an optical system in a disc drive unit must therefore be increased. Further, a magnetic field apply means is necessary in addition to the optical system. These requirements result in a recording/reproduction apparatus of complicated arrangement and high cost.
A phase transition type optical recording medium employs, as its recording layer, a thin film (typically, a thin film containing a chalcogen, e.g., tellurium, selenium, and the like) of a specific material which changes its crystallographic phase between crystalline and amorphous states under certain conditions. A change in birefringence of the recording layer caused by data storage is detected as a change in the reflectance of light, thereby reproducing stored data information. Therefore, the detection sensitivity of reproduction data is necessarily low, and a good reproduction contrast cannot be obtained. As a result, in a practical application, accurate data reproduction cannot be expected.
U.S. Pat. No. 4,404,656 (to Cornet) discloses a bubble mode optical recording medium, wherein a recording layer comprises an organic layer and a metal layer (or alloy layer) formed on an intermediate layer. Upon radiation of a data-writing laser beam, the double-layered recording layer is locally deformed to form a protuberance. More specifically, the upper metal layer is plastically deformed by pressure of a gas released from the organic layer upon heating, thereby forming the protuberance. Since, in a recording medium of this type, plastic deformation of the recording layer is permanent, the data storing protuberance once formed cannot be flattened. In other words, the recording medium disclosed in this patent is not a data-rewritable type.
U.S. Pat. No. 4,371,954 (to Cornet) discloses another bubble mode optical recording medium, wherein a recording layer comprises two layers which have different thermal expansion coefficients and are adhered to each other, e.g., a first layer (a polymer or metal layer) which releases a gas and is deformed thereby upon heating, and a second layer (a shape memory alloy layer) which exhibits a martensitic phase at an ambient temperature. The double-layered recording layer is deformed upon local heating to form a protuberance, thereby storing bit data therein. When the stored data is to be erased, a light beam having a different wavelength is radiated to cancel the protuberance deformation of the shape memory alloy layer. However, with presently existing manufacturing technology, it is very difficult to effect a mass production process wherein the shape memory alloy layer is deposited with its thickness reduced to 100 nanometers or less. Since such a shape memory alloy layer is easy to corrode, even if it can be successfully mass-produced, the resultant recording layer cannot be guaranteed to be stable.